Number of periodic palindromes using exactly four different symbols.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =12a(6) =24a(7) =132a(8) =240a(9) =900a(10) =1560a(11) =4980a(12) =8400a(13) =24612a(14) =40824a(15) =113652a(16) =186480a(17) =502500a(18) =818520a(19) =2158260a(20) =3498000a(21) =9087012a(22) =14676024a(23) =37728372a(24) =60780720a(25) =155091300a(26) =249401880a(27) =632972340a(28) =1016542800a(29) =2569858212